A space launch vehicle typically has two propellant tanks, one for fuel and one for oxidizer, arranged along its longitudinal axis. This arrangement, however, leaves one tank, the second tank, between the first tank and the engine. Therefore, the feedlines that supply propellant from the first tank to the engine must avoid the second tank.
Routing the feedlines to avoid the second tank produces many problems. For example, traditional external routings produce complicated configurations having elbows where propellant can cavitate and increased length that contributes to excessive propellant boil off. In addition, these configurations typically route the feedlines outside of the vehicle's aerodynamic slipstream. The Saturn V ameliorated some of these problems by forming the feedlines integral with the second tank.
Unfortunately, integrating the feedlines with the second tank presents its own set of problems. When a large temperature differential exists between the propellants, propellant in the second tank that is near the feedlines may degrade in quality, and structural problems may develop at the seals between the feedlines and the second tank due to different thermal expansion characteristics of the feedlines, the seals, and the second tank. Also, the engine and the tanks may experience different loads during launch, producing undesirable stresses on the feedlines.